This invention relates to methods for determining a condition of a resistance spotwelding system or a workpiece in the system and, in particular, to methods for determining a condition of a resistance spotwelding system or a workpiece in the system, the system including an electric servomotor used to move welding electrodes together and apply a clamping force to the welding electrodes.
A relatively new kind of actuator, called a servomotor actuator, may replace a pneumatic cylinder commonly found on resistance spotwelding machines. It is useful to understand the prior art of resistance spotwelding and in particular the apparatus used to do resistance spotwelding. Reference is made to Chapters 17 and 19 of the eighth edition of the WELDING HANDBOOK. 
The difference between a conventional resistance spotwelding welding machine and a servomotor-actuated welding machine is the means by which force is applied to the welding electrodes. Instead of a pneumatic cylinder to supply the force, an electric servomotor is used. In order to supply the required force, on the order of 1000 LBS, with a motor small enough to be practical, the servomotor is usually used to drive gearing in the form of a screw or a nut which then supplies the force to the welding electrodes.
The servomotor is controlled by an electronic servo drive, which regulates the current into the motor to regulate its torque and thus the clamping force on the welding electrodes. In addition to controlling torque (and therefore electrode force), the electronic servo drive also measures the position of the motor and/or the screw it drives so it can control both motor speed and position. Thus, a servomotor-actuated welding machine has the capability to position its welding electrodes precisely anywhere between fully closed and fully open positions. In addition, a servomotor-actuated welding machine has the capability to move the welding electrodes at a particular velocity and to measure changes in force as a function of position.
As illustrated in FIG. 2, a servo system traditionally includes a compensator which takes the desired response (usually in terms of a position or velocity of a motor) and compares it to that measured by a sensing system. Based on this measurement, a new input for the motor is calculated. The compensator may be implemented with either analog or digital computing hardware. Since the compensator consists of computing electronics, it rarely has sufficient power to drive the motor directly. Consequently, the compensator""s output is used to command a power stage. The power stage amplifies that signal and drives the motor. There are a number of motors that can be used in servo systems such as DC motors, brushless DC motors, and hybrid stepping motors. To achieve a mechanical advantage, the motor shaft is usually connected to gearing or gears that move the actual load. The performance of the motor is measured by either a single sensor or multiple sensors. Two common sensors, a tachometer (for velocity) and an encoder (for position), are shown. Finally, those measurements are fed back to the compensator.
The servomotor-actuated resistance welding machine itself is not an object of this invention and little explanation of its internal operation is given herein. For information on how a servomotor-actuated resistance spotwelding machine works, refer to U.S. Pat. Nos. 4,670,641; 5,340,960; 5,405,075 and 5,742,022. These patents discuss the operation of a resistance spotwelding gun, which is a type of resistance spotwelding machine. The general principles described therein apply to all types of resistance spotwelding machines.
It is usually desirable, for economic reasons, to speed up the resistance spotwelding process as much as possible. A significant portion of the time required for welding is called xe2x80x9cSqueeze timexe2x80x9d. As the welding gun is closed on the workpiece, squeeze time is the time interval allowed for the welding gun to close and build up force on the welding electrodes. When a pneumatic cylinder is used to supply the force on the welding electrodes, the squeeze time can vary widely due to changes in the incoming plant air supply pressure. Also, it is difficult to measure exactly when the force on the welding electrodes has actually reached the proper value. Friction and inertia of the cylinder and welding gun make measurements of air pressure in the cylinder unreliable to determine when welding force has reached a proper value. A load cell to measure the actual force on the welding electrodes is expensive, fragile, and unreliable.
Therefore, usual practice is to use a fixed squeeze time to allow the welding electrodes to close and attain proper welding force. If this time is set too low, inconsistent welds result. If it is set too high, production speed is slowed.
As many spot welds are made, wear on the welding electrode faces occurs. Depending upon the material being welded, this wear occurs in various ways. With welding materials such as aluminum, the faces of the electrodes contacting the weld area wear down, and due to the generally conical or spherical shape of the welding electrodes, the contact area of the electrodes to the work increases. As this happens, the current and force required for an optimum weld increase, since the larger contact area spreads both the current and applied force over a larger area.
In other material, such as galvanized steel, welding electrode wear occurs differently. Since the zinc coating of galvanized steel has an affinity for copper and the temperature required for welding steel is considerably above the melting point of zinc, the zinc on the part in the weld area forms an alloy (brass) on the surfaces the copper welding electrode which contact the part being welded. As the first few welds are made with new electrodes, the electrodes actually get slightly longer (they xe2x80x9cgrowxe2x80x9d by a few thousandths of an inch) as this zinc accumulates and alloys with the copper. As more welds are made and the brass layer gets thicker, the brass on the faces of the welding electrodes is softened by the heat of the welding process and extrudes out to the sides of the contact faces of the welding electrodes. This has the effect of both increasing the contact area of the electrodes on the surface of the part and wearing the electrode down. Both of these effects increase the current and force required for an optimum weld.
The usual method of compensating for electrode wear is with a xe2x80x9cweld current stepperxe2x80x9d. A weld current stepper is a feature of the welding control wherein the welding current is increased (or, in special cases decreased) to compensate for welding electrode wear and deterioration. One way to implement a weld current stepper is to have the control keep track of the number of welds made and increase (or in some special cases, decrease) the welding current according to the number of welds made. Another method of implementing a weld current stepper is to use electrical measurements to identify events during the welding process and increase or decrease the welding current in response to these events. See U.S. Pat. Nos. 4,104,724; 4,885,451; 5,083,003; 5,386,096 and 5,449,877 for more information in this area.
The present methods of implementing a weld current stepper all have drawbacks and shortcomings. The rate at which welding electrodes wear or deteriorate varies tremendously due to variations in the metal being welded, variations in the electrodes themselves, adequacy of cooling water to the electrodes, and a host of other factors. From one day to the next, the rate at which welding electrodes deteriorate or wear can change by a factor of more than 2. Methods of simply counting welds are clearly inadequate to compensate for these variations. Other methods which measure electrical phenomena to implement a weld current stepper base decisions on effects caused partially by welding electrode wear and partially by other factors, such as material variations. These other factors cause the weld current stepper to respond to things other than wear or deterioration of the welding electrodes, which results in increased process variation.
If a weld is made at the very edge of a part, and only a small portion of the electrode faces are actually in contact with the part, severe weld expulsion will occur and the faces of the welding electrodes may be damaged. During this weld, the electrical measurements taken by the welding control can identify weld expulsion (See U.S. Pat. No. 4,885,451 for an explanation of how this is done).
An object of the present invention is to provide a method for determining a condition of a resistance spotwelding system or a workpiece in the system to overcome some of the problems which occur when resistance spotwelding sheet material together.
Another object of the present invention is to provide a method for determining a condition of a resistance spotwelding system or a workpiece in the system and, in response, change parameters of the welding process itself or identify problems to a welding machine operator.
It is another object of the present invention is to provide a method for determining a condition of a resistance spotwelding system or a workpiece in the system to improve control of the spotwelding process.
Still another object of the present invention is to provide a method for determining a condition of a resistance spotwelding system or a workpiece in the system to detect excess friction in a welding machine of the system such as a welding gun.
Yet another object of the present invention is to provide a method for determining a condition of a resistance spotwelding system or a workpiece in the system to allow welding electrode wear to be measured more directly.
Another object of the present invention is to provide a method for determining a condition of a resistance spotwelding system or a workpiece in the system to identify welding problems and reduce process variation.
In carrying out the above objects and other objects of the present invention, a method is provided for determining a condition of a resistance spotwelding system or a workpiece in the system, the system includes a welding transformer, a welding machine, a pair of welding electrodes supported for movement by the welding machine and an electric servomotor adapted to receive electrical inputs to rotate a drive shaft of the servomotor to, in turn, move the welding electrodes together and apply a force to the welding electrodes. The method includes sensing a change in at least one of the electrical inputs and providing at least one signal in response thereto. The method also includes processing the at least one signal to determine the condition.
The condition may be whether the welding electrodes have contacted either the workpiece to be welded or, in the absence of a workpiece, each other.
The at least one of the electrical inputs may be voltage across the servomotor. The method may include sensing changes in current through the welding transformer and generating a corresponding current signal in response thereto. The step of processing processes the at least one signal and the current signal to determine the condition.
The method may include sensing changes in voltage across the welding electrodes and providing a voltage signal in response thereto. The step of processing processes the voltage signal together with the at least one signal to determine the condition.
The at least one of the electrical inputs may be current through the servomotor. The method may include sensing changes in welding transformer current and providing a corresponding current signal in response thereto. The step of processing processes the current signal together with the at least one signal to determine the condition.
The method may include sensing changes in voltage across the welding electrodes and providing a corresponding voltage signal in response thereto. The step of processing then processes the voltage signal together with the at least one signal to determine the condition.
When the electrical inputs are voltage across the servomotor and current through the servomotor, the step of sensing may include the step of sensing changes in both the voltage and the current to provide corresponding voltage and current signals in response thereto. The step of processing processes both of the voltage and current signals to detect that the welding electrodes have contacted either the workpiece to be welded or, in the absence of a workpiece, each other.
The method may include sensing a change in welding transformer current and providing a corresponding transformer current signal in response thereto. The step of processing processes the transformer current signal together with the current and voltage signals to determine the condition.
The method may include sensing a change in voltage across the welding electrodes and providing a corresponding electrode voltage signal in response thereto. The step of processing processes the electrode voltage signal together with the current and voltage signals to determine the condition.
When the condition is a value for mechanical friction of the welding machine as the welding electrodes are closed on each other. The electrical inputs are voltage across and current through the servomotor wherein the step of sensing senses changes in both the voltage and the current to provide corresponding voltage and current signals in response thereto. The step of processing processes the voltage and current signals to determine the value. The method may then further include displaying the value.
The method may further include comparing the value to predetermined limits and producing an error signal when the value is outside the predetermined limits.
The condition may be that the force on the welding electrodes has reached a desired value. The at least one of the electrical inputs may then be voltage across the servomotor. The method may then further include positioning a workpiece between the welding electrodes and applying a welding current to the welding electrodes after determining the condition.
The at least one of the electrical inputs may be current through the servomotor. The method may then further include positioning a workpiece between the welding electrodes and applying a welding current to the welding electrodes after determining the condition.
Both voltage across the servomotor and current through the servomotor may be sensed in the step of sensing and corresponding current and voltage signals provided in response thereto. Both the current and voltage signals are processed in the step of processing to determine the condition. The method may then further include positioning a workpiece between the welding electrodes and applying a welding current to the welding electrodes after determining the condition.
Still further in carrying out the above objects and other objects of the present invention, a method is provided for determining a condition of a resistance spot welding system or a workpiece in the system. The system includes a welding transformer, a welding machine, a pair of welding electrodes supported for movement by the welding machine and an electric servomotor adapted to receive electrical inputs to rotate a drive shaft of the servomotor to, in turn, move the welding electrodes together and apply a force to the welding electrodes. The method includes sensing rotary position of the drive shaft and providing a feedback signal in response thereto. The method also includes processing the feedback signal to determine the condition.
The condition may be a position of the servomotor which corresponds to the welding electrodes being fully in contact with each other without a workpiece between them.
The method may further include positioning at least one workpiece between the welding electrodes wherein the condition is thickness of an area where a weld is desired on the workpiece. The method may further include changing welding parameters of the system based on the thicknesses. The method may further include comparing the thickness to predetermined limits and alerting an operator of the system when the thickness is outside the predetermined limits.
The method may further include positioning a pair of workpieces between the welding electrodes and increasing the force on the welding electrodes. The condition is at least one gap between the workpieces. The steps of sensing and processing are also performed to measure movement of the welding electrodes after the welding electrodes are closed and after the force on the welding electrodes is changed. The method may further include further increasing the force on the welding electrodes to close the at least one gap.
The method may further include positioning at least one workpiece between the welding electrodes and applying welding current to the welding electrodes wherein the steps of sensing and processing are performed as the welding current is applied. The condition may be the absence of a workpiece between the welding electrodes or wear of the welding electrodes.
The condition may be a welding condition which may damage the welding electrodes. The method then further includes positioning at least one workpiece between the welding electrodes, applying a welding current to the welding electrodes and generating at least one electrical signal based on the welding current. The step of processing processes the feedback signal and the at least one electrical signal to determine the condition.
The method may further include positioning a workpiece between the welding electrodes and applying a welding current to the welding electrodes to form a weld wherein the condition is welding electrode indentation into the weld. If the condition is electrode wear, the method may further include adjusting a weld current stepper based on the wear, changing weld current magnitude based on wear, changing weld current duration based on the wear, changing force applied to the welding electrodes based on the wear, or producing an indication when the wear exceeds a predetermined limit.
The condition may be thickness of a workpiece being welded. In this case, the method further includes positioning the workpiece between the welding electrodes, applying a welding current to the welding electrodes to form a weld on the workpiece at a first location, moving the welding electrodes relative to the workpiece to a second location on the workpiece, applying a welding current to the welding electrodes at the second location, performing the steps of sensing and processing to obtain a second thickness, computing a moving average of the weld thicknesses and using the moving average of weld thicknesses to infer an amount of wear of the welding electrodes.
The condition may be apparent change in a fully closed position of the welding electrodes caused by electrode replacement or electrode cleaning.
In the case of electrode replacement, the method may further include resetting a weld current stepper based on the apparent change. The method may further include producing an indication that a weld current stepper needs to be reset based on the apparent change.
In the case of electrode cleaning, the method may further include producing an indication that a weld current stepper needs to be reset based on the apparent change. The method may further include partially canceling a welding current increase caused by a weld current stepper based on the apparent change.
The method may further include positioning a workpiece between the welding electrodes wherein the condition is that the welding electrodes are not making contact with the workpiece. The method further includes energizing the welding transformer at a relatively low power level.
In this case, the method further includes causing an error indication if, while the welding electrodes are not making contact with the workpiece, current in the welding transformer is above a first predetermined value. The method may further include measuring voltage across the welding electrodes and causing an error indication if, while the welding electrodes are not making contact with the workpiece, current in the welding transformer is above a first predetermined value and voltage across the welding electrodes is below a second predetermined value.
The method may further include activating the servomotor to bring the welding electrodes together. In this case, the method may further include verifying that, before the welding electrodes make contact with the workpiece, current in the welding transformer is below a third predetermined value. The method may further include measuring the voltage across the welding electrodes and verifying that, before the welding electrodes make contact with the workpiece, current in the welding transformer is below a third predetermined value and the voltage across the welding electrodes is above a fourth predetermined value.
The method may further include causing an error indication if, before the welding electrodes make contact with the workpiece, current in the welding transformer is above a first predetermined value. In this case, the method may further include measuring changes of power factor in a primary circuit of the welding transformer to obtain measurements and utilizing the measurements to infer location of a short circuit which causes the current in the welding transformer to be above the first predetermined value.
The method may further include causing an error indication if, before the welding electrodes make contact with the workpiece, voltage across the welding electrodes is below a second predetermined value. In this case, the method may further include measuring changes of power factor in a primary circuit of the welding transformer to obtain measurements and utilizing the measurements to infer location of a short circuit which causes the voltage across the welding electrodes to be below the second predetermined value.
The method may further include measuring the voltage across the welding electrodes to obtain a voltage measurement and causing an error indication if, before the welding electrodes have made contact with the workpiece, current in the welding transformer is above a first predetermined value and the voltage measurement is below a second predetermined value. In this case, the method may further include measuring changes of power factor in a primary circuit of the welding transformer to obtain measurements and utilizing the measurements to infer location of a short circuit which causes the current in the welding transformer to be above the first predetermined value and the voltage measurement to be below the second predetermined value.
The method may further include measuring power factor in a primary circuit of the welding transformer to obtain a measurement and causing an error indication if, before the welding electrodes have made contact with the workpiece, the measurement is outside predetermined upper and lower limits. In this case, the method may further include utilizing the measurement to infer location of a short circuit which causes the power factor in the primary circuit of the welding transformer to be outside of the predetermined upper and lower limits.
The method may further include causing an error indication if, after the welding electrodes have made contact with the workpiece, current in the welding transformer is not above a predetermined value. The method may further include measuring voltage across the welding electrodes to obtain a voltage measurement and causing an error indication if, after the welding electrodes have made contact with the workpiece, the current in the welding transformer is not above a predetermined value or the voltage measurement is not below a predetermined value.
The method may further include positioning a workpiece between the welding electrodes and applying a welding current to the welding electrodes to make a weld to the workpiece wherein the steps of sensing and processing are performed to determine indentation of the welding electrodes into the workpiece during application of the welding current. In this case, the method may further include terminating the welding current based on the indentation, measuring electrical signals based on the welding current during application of the welding current and terminating the weld current based on the indentation and the electrical signals, identifying weld expulsion based on the indentation, measuring electrical signals based on welding current during application of the welding current and identifying weld expulsion based on indentation and the electrical signals, or determining when to release the welding electrodes from the weld based on the indentation.
The method may further include positioning the workpiece between the welding electrodes and applying a welding current to the welding electrodes wherein the steps of sensing and processing are performed to determine the condition in which the welding electrodes have stuck to the workpiece as the welding electrodes are being removed from the workpiece after application of the welding current. The method may further include positioning a workpiece between the welding electrodes and applying a welding current to the welding electrodes wherein the steps of sensing and processing are performed to determine the condition in which the welding electrodes have stuck to the workpiece as the welding electrodes are being removed from the workpiece after application of the welding current. In this case, the at least one of the electrical inputs is voltage across or current through the servomotor. Alternatively, the step of sensing senses changes in both voltage across the servomotor and current through the servomotor and both the voltage and current signals are processed in the step of processing to determine the condition.
The capabilities of the servomotor-actuated welding machine to measure and control welding electrode force and position allow the welding controller to gain information that is useful to control the resistance spotwelding process. Although the example of a servomotor actuator on a portable resistance spotwelding gun, one of many types of resistance spotwelding machines, is used herein, the methods of this invention are applicable to any resistance spotwelding machine which uses a servomotor actuator to bring the welding electrodes together and apply force to them.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.